1. Field of the Invention
The present invention relates to a decelerating device. More particularly, the present invention relates to a projection screen rolling mechanism comprising the decelerating device.
2. Detailed Description of the Related Art
A present optical projector, such as a single beam projector and an LCD projector, may project images with high definition and high brightness. In addition, the optical projector has the advantages of lower price which is similar to that of the conventional TV sets, small volume, and easy to carry. Accordingly, the optical projector is increasingly accepted by most people in the society.
On a large occasion for speech, the optical projector projects images on a projection screen such that the audience looks at the information on the projection screen easily. In the conventional art, the projection screen and a rolling unit together compose a projection screen rolling mechanism. One side of the projection screen is connected to a rotation shaft of the rolling unit and the projection screen rolls around the rotation shaft such that another side of the projection screen is exposed. When a user wants to use the projection screen, the user pulls the exposed side of the projection screen such that the projection screen extends gradually and a torsional spring of the rolling unit connected to the rotation shaft deforms elastically. Next, the user fixes the exposed side of the projection screen such that the optical projector can project images on the projection screen.
When the user wants to roll up the projection screen, the user frees the projection screen and the recovering force of the torsional spring drives the rotation shaft to rotate such that the projection screen rolls. However, in the conventional art, the rotational speed of rotation shaft is very fast such that the projection screen is easily damaged.